The present invention relates to the art of adhesive bonding, and more particularly, to the bonding of surfaces wherein the control of bond line thickness is desirable, as for example, in the art of packaging semiconductor devices.
Adhesive pastes have been used in the semiconductor packaging industry in a variety of applications. Epoxy pastes, for example, have been used to secure heat sinks to ceramic semiconductor packages. In addition, metallic-glass pastes have been used for attaching semiconductor chips to ceramic substrates in the manufacture of semiconductor devices, and particularly, large scale integrated (LSI) and very large scale (VLSI) circuits. These circuits typically include a semiconductor chip or die, e.g., silicon, gallium arsenide, etc., which is bonded to a supporting ceramic substrate. Commonly assigned U.S. Pat. Nos. 4,636,254, 4,761,224 and 4,968,738 are directed to improved silver-glass die attach pastes for the attachment of a semiconductor die to a ceramic substrate. These patents disclose silver-glass die attach pastes consisting generally of a mixture of silver flake and glass frit distributed in an organic vehicle including a suitable organic resin and a suitable organic solvent. To form a bond between a semiconductor die and a ceramic wafer substrate, a selected quantity of the silver-glass paste is placed between the opposing die and substrate surfaces in a sandwiched relationship. The sample is dried and fired in a oven or furnace at temperatures above the glass transition temperature (T.sub.g) of the glass constituant. During the drying and firing stages, the organic vehicle volatilizes and the glass flows to wet the ceramic substrate and die, while the silver flake sinters together. Upon cooling, the result is a secure bond between the die and the substrate.
One critical aspect of the semiconductor die/ceramic substrate bonding process is that the post-fired adhesive film must have an adequate bond line thickness. If the bond line thickness is sufficient, the resultant bond will exhibit good resistance to differential thermal expansion rates between the die and substrate and will produce a bond having a high tensile strength. Interfacial stress that arises from a thermal mismatch between the die and substrate is directly proportional to both the area of attachment and the modulus of elasticity of the bonding adhesive. This stress is also inversely proportional to the bond line thickness. Since the modulus of elasticity for a given die attach adhesive is fixed, the only avenue available to reduce interfacial stress is to maintain a sufficient bond line thickness on each part assembled. For the proper bonding of silicon dice to ceramic substrates, the bond line thickness must be increased proportionally to the area of the surfaces to be bonded.
It is a characteristic of metal-glass pastes that the paste collapses during drying and firing as the organic materials are eliminated from the composition. The amount of shrinkage varies from case to case. However, if a sufficiently large wet bond line thickness is initially maintained, post-fired bonds exhibiting suitable strength and thermal stress resistance characteristics may be obtained.
In production scale semiconductor packaging operations, the control of wet bond line thickness has proven problematic. In some cases, spacing control equipment is not always available. In other cases, the equipment, although available, is not always useable (as, for example, in packages with minimal spacing between the die and the cavity wall). This inability to adequately control wet bond line thickness may result in production runs wherein the final (fired) bond line thickness is inadequate. These items may fail adhesion testing, in which case they must be discarded. To overcome this problem, manufacturers have resorted to applying excess amounts of paste to the die-substrate interface. This, of course, needlessly increases the amount of paste utilized, and significantly extends the required processing time.
Accordingly, a practical solution to the problem of bond line control in the packaging of semiconductor devices would be desirable. Of course, the bond line thickness problem is not limited to metal-glass pastes alone, but may be experienced to varying degrees in other compositions such as epoxy and the like. Thus, a bond line control mechanism for applications utilizing epoxy and other adhesives is also required.